1. Field of the Invention
This invention relates to a static pressure bearing device for use in an XY stage of a semiconductor exposing apparatus, or the positioning stages of a precision machine tool and a precision measuring machine or the like.
2. Related Background Art
The XY stage of a semiconductor exposing apparatus and the positioning stages of a precision machine tool and a precision measuring machine or the like require highly accurate positioning and for this purpose, use is made of a static pressure bearing device for noncontactly supporting or guiding the XY stage or the positioning stage relative to a bed plate or the like.
The bearing characteristic of static pressure bearing devices greatly differ depending on the type of restrictor spouting pressurized gas used, and particularly, the frequency characteristic of the pressure fluctuation of a bearing gap when vibration in the XY stage or the positioning stage is generated is substantially constant in a static pressure bearing device of the conventional surface restrictor type or the orifice restrictor type, and the rigidity or the like thereof is not greatly varied by the vibration of the XY stage or the positioning stage. However, a static pressure bearing device of the porous restrictor type, as compared with the surface restrictor type or the orifice restrictor type, has the advantage that high bearing rigidity is obtained even if the amount of supply of pressurized fluid is small, while bearing rigidity may be remarkably reduced or the static pressure bearing device may cause self-excited vibration when vibration is generated in the positioning stage or the XY stage.
FIGS. 10 and 11 of the accompanying drawings are graphs obtained by measuring the pressure fluctuation of gas in the bearing gap of a static pressure bearing device of the porous restrictor type when the dimension of the bearing gap varies periodically, detecting a pressure fluctuation component (hereinafter referred to as the "Re component") of the same phase as the periodical reduction in the dimension of the bearing gap and a pressure fluctuation component (hereinafter referred to as the "Im component") having a phase delay of 90.degree. relative thereto, and obtaining the frequency characteristics of these components. As can be seen from these figures, when for example, the natural frequency of the XY stage or the positioning stage is 150-160 Hz, the Im component is of a negative value and therefore, self-excited vibration as shown in FIG. 12 of the accompanying drawings is generated and as a result, the positioning accuracy of the XY stage or the positioning stage may be remarkably reduced, and the apparatus may be damaged by increasing vibration.
Accordingly, by using a static pressure bearing device of the porous restrictor type in which a predetermined amount of clogging is generated in advance to thereby vary the bearing characteristic or severely restrict permeability or transmittance of porous material, it is contrived to prevent the self-excited vibration of the static pressure bearing device of the porous restrictor type.